1. Field of the Disclosure
The present disclosure is directed in general to communication systems and methods for operating same. More particularly, embodiments of the disclosure provide improved systems and methods for encoding and decoding information transmitted using multiple-input-multiple-output (MIMO) communication devices.
2. Description of the Related Art
Orthogonal space-time block codes (OSTBC) have traditionally been used to achieve a full diversity gain in MIMO communication channels. Maximum likelihood decoding for OSTBC requires linear processing of the received signals. As is known to those of skill in the art, the Alamouti code is an effective OSTBC for channel with two transmit antennas. Alamouti codes can transmit two independent symbols from two antennas in two symbol durations using the code structure shown below in Matrix 1.
                    (                                                            a                1                                                                    -                                  a                  2                  *                                                                                                        a                2                                                                    a                1                *                                                    )                            Matrix        ⁢                                  ⁢        1            
In the above symbol structure symbols a1 and a2 are transmitted from transmit antenna Tx1 and Tx2, respectively, during the first symbol time, and symbols −a*2 and a*1 transmitted from transmit antennas Tx1 and Tx2, respectively, during the second symbol time. The information rate of Alamouti code is log2|A| A bits per transmission when symbols i and belong to constellation sets A and |A| is the number of elements in set A. The detection decision is made only after processing signals received over two symbol times, resulting in a two symbol time decoding delay.
For a large number of transmit antennas, OSTBC code structures are complex and are not able to achieve a full information rate. One well known OSTBC for four (referred to herein as OSTBC-4) transmit antennas has the code structure shown below in Matrix 2.
                    (                                                            a                1                                                                    -                                  a                  2                  *                                                                                    -                                  a                  3                  *                                                                    0                                                                          a                2                                                                    a                1                *                                                    0                                                      -                                  a                  3                  *                                                                                                        a                3                                                    0                                                      a                1                *                                                                    a                2                *                                                                        0                                                      a                3                                                                    -                                  a                  2                                                                                    a                1                                                    )                            Matrix        ⁢                                  ⁢        2            
In the above code, four symbol periods are required to transmit three symbols, thus requiring an information rate of
      3    4    ⁢      log    2    ⁢          A      bits per transmission. Another drawback of the code shown in Matrix 2 is that the channel should remain constant for four symbol blocks and, therefore, the code may have limited applicability in high Doppler channels. Furthermore, the decoding delay for the code structure shown in Matrix 2 is also of four symbol periods.
Current encoders and detectors for implementing the coding discussed above in Matrix1 and Matrix 2 is comparatively processor intensive and result in significant decoding delay. It would be desirable, therefore, to implement a code structure that provides a high degree of accuracy while minimizing the use of processor resources and also minimizing decoding delay. Improved systems and methods for encoding and decoding information transmitted using multiple-input-multiple-output (MIMO) communication devices to meet the aforementioned needs are disclosed herein.